What kind of iron do you have in there?
Those lytic caps I sent you are pretty nice, I'd also recommend 68uf/450v Panasonic ED's in place of the 22uF/450V and 100uF/450V power supply caps, they will also leave you more room than the oilers. Note that this recommendation is only for ease of construction, the stock capacitors in the SEX amp are beefy and durable, but give less flexibility during construction.
One could also use a TL431 to bias the driver stage, if you have the C4S upgrade boards installed, I can walk you through that one (or see the original Paramount 300B circuit).
If you're going through the trouble of adding a Hammond box, I'd add a second power transformer and shunt regulate the whole thing, but that's probably more than you want to work on.
-PB